1. Field of the Invention
The present invention relates to a method of forming a circuit such as a printed circuit board and an apparatus therefor.
2. Related Background Art
As advances are made in electronic and visual equipment such as a computer and a video, demand for printed circuit boards is increased and presently, printed circuit boards form a large market accounting for 10% of the whole electronic equipment market. Naturally, demand for printed circuit board forming apparatuses is high.
Usually, a printed circuit board is made by forming a circuit pattern as an electric wiring diagram by means of a good electric conductor such as copper on an insulating board. In order to form the circuit pattern, an additive or subtractive method is utilized. In the additive method, the circuit pattern is formed directly on the board by the use of metallic foil. On the other hand, in the subtractive method, the circuit pattern is formed on the board by coating the board with a metallic foil layer, forming a resist pattern thereon, and removing an area of the metallic foil layer other than the resist pattern by means of etching.
For forming the resist pattern according to the subtractive method, there are mainly an exposure method and a screen printing method. In the exposure method, the resist pattern is formed by coating the board with a photoresist layer, and exposing and developing the photoresist layer via a mask pattern corresponding to the circuit pattern. On the other hand, in the screen printing method, the resist pattern is directly printed through a stencil screen corresponding to the circuit pattern. As the recent trend of the wiring technique, multiple-layer boards have increased in number, and more minute patterns are required. Also, thin and flexible boards have increased in number.
Thus, presently, the screen printing and the lithography by means of the exposure are mainly used to form the resist pattern for forming the circuit pattern of the printed circuit board. However, in the screen printing, its process is simple, but there is still a limit to a pattern width (about 100 .mu.m) and stable formation of the circuit pattern is difficult. Therefore, when a more minute pattern width is required, the lithography is used. However, the number of its processes is large and the photoresist is expensive.
In either way, the mask pattern or the stencil screen (hereinafter referred to as the mask) is required to form the circuit pattern. In this case, the formation of the mask is troublesome. Also, the usage of the mask leads to an increase in the number of inspection processes and an increase in the rate of defective products when the mask has defects. Further, these processes are not necessarily economical when forming a small amount of products.
In view of these problems, it is considered to draw the resist pattern directly without utilizing the mask. For example, a drawing apparatus for scanning and exposing a photoresist with an Ar laser and an apparatus for forming a pattern directly by spraying fine-grained metal are considered. However, problems are still left in terms of throughput and cost. In such a laser scan drawing apparatus, focusing is difficult because of irregularities of the board. Also, the sensitivity of the photoresist is not sufficient, so a high-power laser is required in the drawing and the drawing speed is lowered.